Granulated carbon microphone



May 20, -1947 J. DE B'OER GRANULATED CARBON MICROPHONE Filed March 23 1945 3 R 4410.4/7005 3 w v l l 500 000 2000 3000 5000 10,000 (YCZES i BY wzw ATTORN EY Patented May 20, 1947 GRANULATED CARBON MICROPHONE Jan de Boer, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn, as trustee Application March 23, 1943, Serial No. 480,193 In the Netherlands November 11, 1939 1 Claim. 1

This invention relates to a granulated carbon microphone of the type which is to be placed against a part of the human body close to the vocal chords in order to derive therefrom the acoustic vibrations which are to be translated into electrical oscillations. It is known to place such microphones against the throat, against the cheek, or against the skull but also in the oral cavity. The housing of the microphone contacts with the skin and the vibrations produced by the voice are transmitted through the skin to the housing or to the diaphragm.

The microphone according to the invention, which is equipped in the usual way with a, granulated carbon chamber closed by the diaphragm is characterized in that the diaphragm is completely shut off from the direct driving force and the mass freely following the vibrations of the diaphragm is so chosen that due to the inertia it is caused to move by the vibrations of the housing to a, sufficient degree to effect the translation of acoustic vibrations into electrical oscillations.

Since the diaphragm is moved exclusively by inertia, it is not brought into contact with the skin but it derives its vibrations solely from the movements of the housing. To give the diaphragm sufficient mass, a weight freely following the movements of the diaphragm may be placed on the moving parts thereof. If the diaphragm is clamped at its edge in the housing the center of this diaphragm, upon vibration of the housing, will perform movements corresponding to the said vibrations due to its inertia.

The invention will be more fully explained by reference to the accompanying drawing, representing, by way of example, one embodiment thereof.

Fig. 1 is a section of a granulated carbon microphone; and

Fig. 2 represents the amplitude frequency curve of such a microphone.

In Fig. 1, the reference number l designates the housing of the microphone consisting of synthetic resin or a similar insulating material. A recess II in the housing forms a chamber accommodating the carbon granules, an electrode shaped as a gilded brass disc 12 forming part of the boundary of this granulated carbon chamber. This brass electrode I2 is furnished with a wire end l3 which constitutes a connecting terminal. The other boundaries of the granulated carbon chamber I I is constituted by the circular diaphragm M which is clamped solely at its outer edge ll between the bottom part II] and the cover I5 of the housing. By means of the screw look It, the cover l5 makes a substantially airtight fit with the part It of the housing so that direct driving of the diaphragm M by vibrations of the air is impossible. Therefore, the driving of the diaphragm takes place exclusively by inertia, i. e. the housing of the diaphragm, more particularly the dome shaped cover i5, is pushed against the part of the body vibrating in accordance with the vibrations of the voice as a result of which the whole housing is made to vibrate. Solely the outer rim ll of the diaphragm i t is secured to the housing. The remaining part of the diaphragm is not by any means connected to the housing or to a driving member projecting from the housing so that the driving is sole- 1y brought about by the inertia of the diaphragm, itself. However, the movable part lags with respect to the clamped part.

The value of the amplitude difference between the diaphragm and the housing and more particularly of the part which is most freely movable, i. e. the central part, is proportional to the product of the value of the mass of the non-clamped part of the diaphragm and of the acceleration imparted into the housing as long at least as the natural resonance of the clamped diaphragm has not yet been attained. An increase in amplitude can be achieved by coupling a separate mass with the diaphragm. To this end a lead weight 18 is secured to the diaphragm at its center. Above the resonance frequency the diaphragm is at rest in space. In the range of frequencies below this resonance frequency, the amplitude differences of th diaphragm and housing at different frequencies are related as the accelerations, i. e. as the squares of the frequencies. Hence, in the range of frequencies below resonance, the amplitude-frequency curve shown in Fig. 2 exhibits a curve 30 which rises towards the higher frequencies.

At 3|, the natural resonance of the clamped diaphragm is attained at a frequency of about 2700 cycles/sec. In fact, it is imperative for the intelligibility of the human voice that the sound frequencies should be perfectly reproduced up to at least 1500 cycles/sea, whereas the frequencies exceeding 3000 cycles/sec. are substantially of no importance. In order to render the amplifying apparatus and reproducing devices not unnecessarily expansive with installations designed solely for the transmission of speech, no value is attached to a frequency curve having still a horizontal part above 3000 cycles/sec. For this reason, it is sufficient for a microphone of such 3 design that the resonance lies between 1500 and 3000 cycles/sec.

It has been found that the favorable value of the height of the granulated carbon chamber in the state of rest in the direction in which the compression is effected by the diaphragm cor responds to about ten times the maximum amplitude occurring, the resistance of the layer yielding the maximum resistance variations without leaving the linear relation existing between amplitude and resistance. Again, it has been found that the maximum amplitude of the skin at the larynx with a normal voice is about 0.1 mm. at 300 cycles/sec. The amplitude of the microphone housing pressed against the larynx has the same value. Consequently, the height of the granulated carbon chamber should be about 1 mm. Hence, the dotted curve 32 indicates the amplitudes of the housing in accordance with the frequency. It appears from the two curves 30 and 32 that the effective output energy of the microphone well approaches a horizontal line in the range below 3000 cycles/sec.

The diaphragm vibrates in a manner similar to a beam clamped at both ends, i. e. the central part has the largest amplitude. Consequently,

the outermost part of the diaphragm is less oper-- ative than the central part. For this reason, the recess II is only partly filled with carbon granules about to half the section of the diaphragm. The outermost part is not used as a carbon chamber and is filled up with a ring l9 of wadding or similar material capable of being easily compressed. The surface of the gilded brass electrode I2 is also limited to the dimensions of the layer of carbon. The diaphragm l4 acts as a counterelectrode. The electrical connection from the diaphragm to the connecting terminal 2! is constituted by a brass rod 20 embedded in the housing H), which rod locally contacts with the clamped part I! of the diaphragm.

The carbon chamber II is filled, after providing the diaphragm l4, through a filling aperture 23 in the housing l0 and a bore 22 corresponding therewith in the electrode l2. This filling aperture is closed by means of a prop of wadding 24.

What is claimed is:

A microphone of the inertia type to be placed against the human body and using granulated carbon, comprising a casing member, a recess in said casing forming a chamber for said granulated carbon, a flat disc metallic diaphragm forming an electrode for said microphone and covering over said granulated carbon which is located in said recess, the dimensions of said recess in the direction of compression of said granulated carbon being in the order of magnitude of ten times the maximum amplitude of the diaphragm, a solid cylindrical lead weight secured to the central portion of said diaphragm to increase the amplitude thereof, another electrode located in the lower portion of said recess and in contact with said granulated carbon, a cover for completely enclosing said casing whereby the microphone is responsive to the natural resonance in the proximity of the upper limit of the range of audio frequencies.

JAN DE BOER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,932,581 Gilman Oct, 31, 1933 354,241 Rose Dec. 14, 1886 1,149,610 Allis Aug. 10, 1915 2,260,727 Sears et al Oct. 28, 1941 2,340,777 Stanley Feb. 1, 1944 

